A couple of reasons come to mind.
Most of the SD channels you name are actually available in other packages already, and those are the only ones in SD in the Platinum pack. Many customers wouldn't be gaining anything much by paying $10 for them. I can just imaging the irate phone calls after signing up and finding most all of the channels are not available. So that would be why a HD receiver.
This package is considered a Tier, not meant to replace getting HD programming. Since HD is now free, you are not really being made to pay for HD before getting this tier anyway. Remember, it was originally promoted as an HD add on, which really it still mostly is. As a whole, this is a decent value package.